X-irradiation of the whole head of the newborn rat results in growth retardation, the degree of which depends on the age of the animal and the dose of radiation. The present experiments were carried out in order to further define the mechanism involved in the growth impairment. Long-Evans black hooded rats (1126) were either irradiated or used as littermate controls. The areas of the head that were irradiated were (1) whole head, (2) right and left halves, (3) lateral outer zones, (4) transverse bands in anterior, middle, and posterior locations, and (5) midsagittal bands. The rats were all irradiated at 2 days of age except one small group which was irradiated at 1 day of age. The radiation doses ranged from 600 to 2000 R. The results indicate that bilateral irradiation of structures near the midline in the general area of the hypothalamus will produce growth retardation. Irradiation of the pituitary gland is not required for the growth failure. Evidence against the possibility that the growth disturbance is caused by a disturbance of well-known endocrine functions or nutritional deficiency is summarized. The implications of the present findings in the problem of growth controls are discussed.

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